1. Field of the Invention
This invention relates to a surge arrester of the kind comprising a top terminal, a bottom terminal and at least two surge arrester units, each surge arrester unit comprising an electrically insulating housing, provided with matallic flange means, and a plurality of electrically series-connected metal oxide varistor blocks arranged in a vertical stack in the electrically insulating housing, and the metallic flange means at the joint between each adjacent pair of surge arrester units forming a galvanic connection between the varistor stacks and the outer surfaces of the electrically insulating housings. In particular, but not exclusively, the invention relates to a zinc oxide surge arrester in which the metal oxide varistor blocks comprise zinc oxide varistor blocks.
2. Description of the Prior Art
In contrast to the varistor blocks in a conventional surge arrester having silicon carbide (SiC) blocks and series-connected spark gaps, the varistor blocks in a zinc oxide (ZnO) surge arrester (with or without spark gaps) are continuously subjected to a certain operating voltage when the surge arrester is connected into a network which is under voltage. The surge arrester must be dimensioned so that this voltage stress, to which the ZnO blocks are continuously subjected during normal operation, does not exceed a predetermined value at any place in the surge arrester.
The voltage distribution along ZnO surge arresters of known design is substantially determined by the self-capacitances of the varistor blocks, by the leakage capacitances of the blocks to ground, and by a grading ring normally arranged at the top of the surge arrester. The main purpose of providing a grading ring in a ZnO surge arrester is to improve the evenness of the voltage distribution along the surge arrester which would otherwise be more uneven due to the aforementioned leakage capacitances. However, a completely even distribution cannot be achieved with such a known design, and, accordingly, there is always a higher voltage stress at the upper part of the known design of surge arrester than at the lower part thereof.
The active parts (e.g. metal oxide varistor blocks) of a surge arrester for outdoor use are usually enclosed in an electrically-insulating, porcelain housing with metallic end flanges. For reasons of manufacturing technique, such a porcelain housing cannot be made too long. Surge arresters for voltages higher than about 150 kV are therefore usually built up of two or more surge arrester units mounted on top of each other. Moreover, by constructing a surge arrester with several shorter porcelain housings, a higher short-circuit safety can be obtained since pressure relief can be arranged at each joint between the surge arrester units. However, in these so-called multi-unit surge arresters the leakage capacitances of the joints to earth will contribute further to an uneven distribution of the voltage along the surge arrester, and thus contribute to the top unit becoming relatively more highly stressed than the other lower unit or units.
It has been found that a ZnO surge arrester, consisting of several series-connected surge arrester units, will have an unevenly distributed temperature increase when exposed to long-term external fouling or pollution, e.g. salt deposition on the electrically-insulating housing. This is due to a considerable leakage current flowing along the pollution layer formed on the external surface of the electrically-insulating housing and which in moist condition, is electrically conducting, with the result that there are often great fluctuations in the potential on the metallic flanges of the joints between adjacent surge arrester units. However, by measurements it has been established that, in most cases of heavy external fouling, the pollution layer on the surface of the electrically-insulating housing functions as an unsymmetrical outer control chain which gives a lower relative stress on the surge arrester unit which is placed lowest, that is, a voltage distribution similar to that occurring in the clean condition.
Since the inner active parts of the surge arrester are in galvanic connection with the outer surface of the insulator housing, the voltage stress on the varistor blocks will be affected and may locally rise above the maximum value for which the surge arrester is dimensioned. Because of the great non-linearity (a small difference in voltage gives a great difference in current) of the ZnO blocks, the resistive leakage current through the blocks will then increase temporarily in parts of the surge arrester and cause an abnormal temperature increase in the blocks. Since the resistive leakage current of ZnO blocks is greatly temperature-dependent, the abnormal temperature increase will result in a further increase of the leakage current. This may successively lead to the varistor blocks being destroyed.
Different solutions have been proposed to overcome the above-mentioned problems. One proposal consists of using insulating joint attachments at the joints between the surge arrester units, so that in these places galvanic connection between the active parts of the surge arrester and the outer surface of the insulator housing is avoided. However, such a solution is difficult to apply in a practical design. Another proposal consists of parallel-connecting all the surge arrester blocks with a separate chain of control capacitors to achieve a more even voltage distribution in the same way as in surge arresters having spark gaps and silicon carbide blocks. It is true that such a capacitive control makes it possible to reduce the temperature increase, but, in order to reach acceptably low values, a high capacitance is required, which results in high costs.